Absorbent articles for receiving and retaining bodily discharges such as urine or faeces such as disposable diapers, training pants, and adult incontinence articles are well known in the art, and significant effort has been spent against improving their performance. Such improvements generally aim at addressing the primary function of such articles, namely retaining body fluids, but also at minimizing the negatives associated with wearing such articles by increasing the comfort of the wearer.
Such improvements can mostly be classified to primarily fall within either of two categories: primarily relating to “core technology”, i.e., “absorbency” in the broad sense of the word, or primarily relating to “chassis technology”.
The first addresses how to pick up and retain the body waste (generally in some state of fluidity) in an “absorbent (or core) structure”, whereby the waste material is acquired by the article (picked up), then conducted away from the location of acquisition (distribution), and then stored (retained).
The second category deals—generally—with the so called “chassis elements” to contain the body waste within the confinement of the article. This can be done by separating the absorbent (core) structure and the outside, e.g., wearers' garments or skin, by using an impermeable backsheet. Additionally the chassis should prevent bodily exudates from escaping through the space between the absorbent article and the body of the wearer, which can be achieved by elasticized gatherings at leg and waist openings. Other chassis aspects enable application of the article to the wearer—e.g., by providing closure means such as tapes, or maintain the article on the wearer by belt like arrangements (integral with the article in so called pull-up diaper designs or as part of the application means).
Often the terminology “comfort” for the wearer was predominantly addressed by improving chassis elements, such as by adopting the chassis elements of the diaper to provide good “fit” of the article and to be soft and cushioning. It is also well established that reducing the thickness of the article by reducing the primary thickness cause, i.e., the absorbent (core) structure helps to improve comfort. This however was always a question of balance between liquid handling performance and thickness. Also a substantial amount of cushioning was considered necessary for comfortable diapers. Finally, the skilled person considered it impossible to reduce or even remove the fibrous material to a point where the modern particulate super absorbent materials would take over part or all of the liquid acquisition and distribution functionalities previously provided by fibrous matrixes. Finally, even if there were structures which could possibly provide all such beneficial aspects when dry, it was completely in-conceivable that this could be build into an absorbent (core) structure such that the liquid handling and comfort performance would be maintained even after the first gushes of liquid had been absorbed.
The development of absorbent (core) structures of particular thinness has also other beneficial aspects making such a development the subject of substantial commercial interest. For example, thinner diapers are not just less bulky to wear and fit better under clothing they are also more compact in the package, making the diapers easier for the consumer to carry and store. Compactness in packaging also results in reduced distribution costs for the manufacturer and distributor, including less shelf space required in the store per diaper unit.
As indicated, the ability to provide thinner absorbent articles such as diapers has been contingent on the ability to develop relatively thin absorbent (core) structures that can acquire and store large quantities of discharged body fluids, in particular urine. In this regard, the use of absorbent polymers often referred to as “hydrogels,” “super absorbents” or “hydrocolloid” material has been particularly important. See, for example, U.S. Pat. No. 3,699,103 (Harper et al), issued Jun. 13, 1972, and U.S. Pat. No. 3,770,731 (Harmon), issued Jun. 20, 1972, that disclose the use of such absorbent polymers (hereafter referred to as any of the following: hydrogel forming absorbent polymers, super absorbents, super absorbent polymers or SAPs, absorbent gel materials or AGMs). Indeed, the development of thinner diapers has been the direct consequence of thinner absorbent cores that take advantage of the ability of these SAPs to absorb large quantities of discharged body fluids, typically when used in combination with a fibrous matrix. See, for example, U.S. Pat. No. 4,673,402 (Weisman et al), issued Jun. 16, 1987 and U.S. Pat. No. 4,935,022 (Lash et al), issued Jun. 19, 1990, that disclose dual-layer core structures comprising a fibrous matrix and SAPs useful in fashioning thin, compact, non-bulky diapers.
Prior to the use of these SAPs, it was general practice to form absorbent structures, such as those suitable for use in infant diapers, entirely from wood pulp fluff. Given the relatively low amount of fluid absorbed by wood pulp fluff on a gram of fluid absorbed per gram of wood pulp fluff, it was necessary to employ relatively large quantities of wood pulp fluff, thus necessitating the use of relatively bulky, thick absorbent structures. The introduction of these SAPs into such structures has allowed the use of less wood pulp fluff. These SAPs are superior to fluff in their ability to absorb large volumes of aqueous body fluids, such as urine (i.e., at least about 15 g/g), thus making smaller, thinner absorbent structures feasible. In addition SAP particles typically pack closer than fibrous structures, thus achieving even thinner cores at elevated concentrations.
These SAPs are often made by initially polymerizing unsaturated carboxylic acids or derivatives thereof, such as acrylic acid, alkali metal (e.g., sodium and/or potassium) or ammonium salts of acrylic acid, alkyl acrylates, and the like. These polymers are rendered water-insoluble, yet water-swellable, by slightly cross-linking the carboxyl group-containing polymer chains with conventional di- or poly-functional monomer materials, such as N,N′-methylene-bisacryl-amide, trimethylol-propane-triacrylate or triallyl-amine. These slightly cross-linked absorbent polymers still comprise a multiplicity of anionic (charged) carboxyl groups attached to the polymer backbone. It is these charged carboxyl groups that enable the polymer to absorb body fluids as the result of osmotic forces, thus forming hydrogels.
The degree of cross-linking determines not only the water-insolubility of these SAPs, but is also an important factor in establishing two other characteristics of these polymers: their absorbent capacity and gel strength. Absorbent capacity or “gel volume” is a measure of the amount of water or body fluid that a given amount of SAP will absorb. Gel strength relates to the tendency of the SAP to deform or “flow” under an applied stress. SAPs useful as absorbents in absorbent structures and articles such as disposable diapers need to have adequately high gel volume, as well as adequately high gel strength. Gel volume needs to be sufficiently high to enable the SAP to absorb significant amounts of the aqueous body fluids encountered during use of the absorbent article. Gel strength needs to be such that the SAP formed does not deform and fill to an unacceptable degree the capillary void spaces in the absorbent structure or article, thereby inhibiting the absorbent capacity of the structure/article, as well as the fluid distribution throughout the structure/article. See, for example, U.S. Pat. No. 4,654,039 (Brandt et al), issued Mar. 31, 1987 (reissued Apr. 19, 1988 as U.S. Reissue Pat. 32,649) and U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989.
Prior absorbent structures have generally comprised relatively low amounts (e.g., less than about 50% by weight) of these SAPs. See, for example, U.S. Pat. No. 4,834,735 (Alemany et al.), issued May 30, 1989 (preferably from about 9 to about 50% SAP in the fibrous matrix). There are several reasons for this. The SAPs employed in prior absorbent structures have generally not had an absorption rate that would allow them to quickly absorb body fluids, especially in “gush” situations. This has necessitated the inclusion of fibers, typically wood pulp fibers, to serve as temporary reservoirs to hold the discharged fluids until absorbed by the SAP.
More importantly, many of the known SAPs exhibited gel blocking. “Gel blocking” occurs when particles of the SAP are wetted and the particles swell so as to inhibit fluid transmission to other regions of the absorbent structure. Wetting of these other regions of the absorbent member therefore takes place via a very slow diffusion process. In practical terms, this means acquisition of fluids by the absorbent structure is much slower than the rate at which fluids are discharged, especially in gush situations. Leakage from the absorbent article can take place well before the particles of SAP in the absorbent member are even close to being fully saturated or before the fluid can diffuse or wick past the “blocking” particles into the rest of the absorbent member. Gel blocking can be a particularly acute problem if the particles of SAP do not have adequate gel strength and deform or spread under stress once the particles swell with absorbed fluid. See U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989.
This gel blocking phenomena has typically necessitated the use of a fibrous matrix in which are dispersed the particles of SAP. This fibrous matrix keeps the particles of SAP separated from one another. This fibrous matrix also provides a capillary structure that allows fluid to reach the SAP located in regions remote from the initial fluid discharge point. See U.S. Pat. No. 4,834,735 (Alemany et al), issued May 30, 1989. However, dispersing the SAP in a fibrous matrix at relatively low concentrations in order to minimize or avoid gel blocking can lower the overall fluid storage capacity of thinner absorbent structures. Usage of lower concentrations of these SAPs limits somewhat the real advantage of these materials, namely their ability to absorb and retain large quantities of body fluids per given volume. Another reason why extremely high concentrations of SAP were not possible resides in the physical integrity disadvantage of structures made of particulate material. Creating a fibrous matrix therefore also had the advantage of providing a fiber re-enforced structure, similar to those used in many other technical situations where structural re-enforcement is provided by fibrous elements, such as in fiberglass.
Besides increasing gel strength, other physical and chemical characteristics of these SAPs have been manipulated to improve their performance especially to decrease gel blocking. One characteristic is the particle size, and especially the particle size distribution. In this context it should be mentioned that the present invention relates to absorbent cores comprising particulate SAPs. The use of fibrous SAPs does eliminate many of the problems found with particulate SAPs, but raises different issues. One key problem, which so far has typically led away from commercial usage of fibrous SAP (i.e., regardless of technical aspects), is the cost differential between SAP particles and SAP fibers. According to U.S. Pat. No. 5,047,023 (Berg), issued Sep. 10, 1991 the particle size distribution of the SAP can be controlled to improve absorbent capacity and efficiency of the particles employed in the absorbent structure. However, even adjusting the particle size distribution does not, by itself, lead to absorbent structures that can have relatively high concentrations of these SAPs. Another characteristic of these SAPs that has been looked at is the level of extractables present in the polymer itself; see e.g., U.S. Reissue Pat. 32,649 (Brandt et al.), reissued Apr. 19, 1988.
Yet another characteristic the art has known for some time, as a measure of gel blocking is the Demand Wettability or Gravimetric Absorbence of these SAPs. See, for example, U.S. Pat. No. 5,147,343 (Kellenberger), issued Sep. 15, 1992 and U.S. Pat. No. 5,149,335 (Kellenberger et al.), issued Sep. 22, 1992 where these SAPs are referred to as “superabsorbent materials” and where Demand Wettability/Gravimetric Absorbence is referred to as Absorbency Under Load (AUL). “AUL” is defined in these patents as the ability of the SAP to swell against an applied restraining force (see U.S. Pat. No. 5,147,343, supra, at Col. 2, lines 43-46). The “AUL value” is defined as the amount (in ml./g or g/g.) of 0.9% saline solution that is absorbed by the SAPs while being subjected to a load of 21,000 dynes/cm2 (about 0.3 psi). The AUL value can be determined at 1 hour (see U.S. Pat. No. 5,147,343) or 5 minutes (see U.S. Pat. No. 5,149,335). SAPs are deemed to have desirable AUL properties if they absorb at least about 24 ml./g (preferably at least about 27 ml./g) of the saline solution after 1 hour (see U.S. Pat. No. 5,147,343) or at least about 15 g/g (preferably at least about 18 g/g) of the saline solution after 5 minutes.
AUL as defined in U.S. Pat. Nos. 5,147,343 and 5,149,335 may provide some indication of which SAPs will avoid gel blocking in some instances. However, AUL is inadequate for determining which SAPs will provide the absorbency properties necessary so that the concentration of these polymers in absorbent structures can be increased without significant gel blocking or some other undesirable effect. Indeed, certain of the SAPs disclosed in U.S. Pat. Nos. 5,147,343 and 5,149,335 as having satisfactory AUL values will have inadequate permeability to be useful at high concentrations in absorbent members. In order to have a high AUL value, it is only necessary that the hydrogel layer formed have at least minimal permeability such that, under a confining pressure of 0.3 psi, gel blocking does not occur to any significant degree. The degree of permeability needed to simply avoid gel blocking is much less than that needed to provide good fluid transportation properties. SAPs that avoid gel blocking by AUL selection according to U.S. Pat. Nos. 5,147,343 and 5,149,335 can still be greatly deficient in other fluid handling properties.
Another problem with using AUL values measured according to U.S. Pat. Nos. 5,147,343 and 5,149,335 is that they do not reflect all of the potential pressures that can be operative on the hydrogel-forming polymer in the absorbent structure. As noted above, AUL is measured in these patents at a pressure of about 0.3 psi. It is believed that a much higher confining pressure of about 0.7 psi. more adequately reflects the full range of localized mechanical pressures (e.g., sitting, sleeping, squatting, taping, elastics, leg motions, other tension and torsion motions) on an absorbent structure. See U.S. Pat. No. 5,147,345 (Young et al.), issued Sep. 15, 1992. Additionally, many of the absorbent structures that comprise these SAPs can include other components, such as an acquisition layer that receives the initial discharge of body fluids. See, for example, U.S. Pat. No. 4,673,402 (Weisman et al.), issued Jun. 16, 1987 and U.S. Pat. No. 4,935,022 (Lash et al.), issued Jun. 19, 1990. This acquisition layer can comprise fibers, such as certain chemically stiffened fibers, that have a relatively high capillary suction. See, for example, U.S. Pat. No. 5,217,445 (Young et al.), issued Jun. 8, 1993. To take into account these additional capillary pressures that could affect fluid acquisition by these SAPs, it is more realistic to measure demand absorbency performance under a higher pressure, i.e., approximately 0.7 psi. This would take into better account not only the localized mechanical pressures exerted during use, but also the additional capillary pressures resulting from other components (e.g., acquisition layer) present in the absorbent structure.
For absorbent structures having relatively high concentrations of these SAPs, other characteristics of these absorbent polymers have been evaluated. See, for example, European patent application 532,002 (Byerly et al.), published Mar. 17, 1993, which identifies a characteristic called Deformation Under Load (DUL) as being important for absorbent composites having high concentrations of SAPs. “DUL” is used in European patent application 532,002 to evaluate the ability of the SAP to maintain wicking channels after the absorbent polymer is swollen. See page 3, lines 9-10. DUL values are obtained by incompletely saturating the SAP with a fixed amount of synthetic urine, compressing the absorbent polymer under a light load (0.3 psi), and then measuring the deformation of the absorbent polymer under a heavier load (0.9 psi). In this application SAPs having DUL values of about 0.6 mm or less are disclosed.
DUL as defined in European patent application 532,002 may provide some indication of the ability of SAP to maintain wicking channels after the absorbent polymer is swollen. However, it has been found that the openness or porosity of the hydrogel layer, hereinafter referred to as PHL (for a further definition see PCT WO-95-26209, page 17 following or EP-B-752892 paragraph 64 following), which is formed when these absorbent polymers swell in the presence of body fluids is more relevant than DUL values for determining the ability of these absorbent polymers to acquire and transport fluids, especially when the absorbent polymer is present at high concentrations in the absorbent structure. Porosity refers to the fractional volume that is not occupied by solid material. For a hydrogel layer formed entirely from a SAP, porosity is the fractional volume of the layer that is not occupied by hydrogel. For an absorbent structure containing the hydrogel, as well as other components, porosity is the fractional volume (also referred to as void volume) that is not occupied by the hydrogel, or other solid components (e.g., fibers).
Importantly, it has been found that SAPs having higher porosities than those apparently desired by European patent application 532,002 are particularly suitable for absorbent structures having high concentrations of these absorbent polymers. It is believed that the SAPs having DUL values below about 0.6 mm that are desired by European patent application 532,002 have relatively low porosities.
Another important property at higher concentrations of these SAPs is their permeability/flow conductivity. Permeability/flow conductivity can be defined in terms of their Saline Flow Conductivity (SFC) values. SFC is well established in the art and defined in great detail e.g., in PCT WO-95-26209, page 69 following or EP-B-752892 paragraph 224 following. SFC measures the ability of a material to transport saline fluids, such as the ability of the hydrogel layer formed from the swollen SAP to transport body fluids. Typically, an air-laid web of pulp fibers (e.g., having a density of 0.15 g/cc) will exhibit an SFC value of about 200×10−7 cm3sec/g. By contrast, typical SAPs such as Aqualic L-74 (made by Nippon Shokubai Co., LTD) and Nalco-1180 (made by Nalco Chemical Co.) exhibit SFC values of at most 1×10−7 cm3sec/g. Accordingly, it would be highly desirable to be able to use SAPs that more closely approach an air-laid web of wood pulp fibers in terms of SFC.
Another factor that has to be considered in order to take full advantage of the porosity and permeability properties of the hydrogel layer formed from these SAPs is the wet integrity of the region or regions in the absorbent member that comprise these polymers. For SAPs having relatively high porosity and SFC values, it is extremely important that the region(s) in which polymers are present have good wet integrity. By “good wet integrity” is meant that the region or regions in the absorbent member having the high concentration of SAP have sufficient integrity in a dry, partially wet, and/or wetted state such that the physical continuity (and thus the capability of acquiring and transporting fluid through contiguous interstitial voids/capillaries) of the hydrogel formed upon swelling in the presence of body fluids is not substantially disrupted or altered, even when subjected to normal use conditions. During normal use, absorbent cores in absorbent articles are typically subjected to tensional and torsion forces of varying intensity and direction. These tensional and torsion forces include bunching in the crotch area, stretching and twisting forces as the person wearing the absorbent article walks, squats, bends, and the like. If wet integrity is inadequate, these tensional and torsion forces can potentially cause a substantial alteration and/or disruption in the physical continuity of the hydrogel such that its capability of acquiring and transporting fluids into and through the contiguous voids and capillaries is degraded, e.g., the hydrogel layer can be partially separated, fully separated, have gaps introduced, have areas that are significantly thinned, and/or broken up into a plurality of significantly smaller segments. Besides the macroscopic discomfort such lack of integrity inevitably creates it can also minimize or completely negate any advantageous porosity and permeability/flow conductivity properties of the SAP. In order to evaluate the behavior of a hydrogel layer a so-called ball burst evaluation can be made allowing predicting or drawing conclusions about the expected behavior within certain limits of the absorbent article in use. In particular the test allows evaluating the relative deterioration of the wet integrity of absorbent cores comprising a high SAP concentration
Accordingly, it would be desirable to be able to provide an absorbent member comprising: (1) a region or regions having a relatively high concentration of SAP particles; (2) with relatively high porosities, and preferably permeability/flow conductivity properties more like an air-laid fibrous web; (3) that can readily acquire fluids from even high capillary suction acquisition layers under typical usage pressures; (4) in a matrix that provides sufficient wet integrity such that its capability for acquiring and transporting fluids is not substantially reduced or minimized, even when subjected to normal use forces. It would also be highly desirable to be able to use SAPs in these absorbent members that, when swollen by body fluids, continue to have a good wet integrity and high porosity such that: (a) the void volume per unit weight of absorbent polymer is closer to that of an air-laid fibrous web; (b) the demand wet ability or gravimetric absorbency of the absorbent polymer under usage pressures is increased; and (c) the absorbent polymer preferably has increased permeability, improved wicking and/or improved swelling properties.
Hence it is an object of the present invention to provide absorbent articles having an improved fit especially by reducing their thickness but also when being loaded, together with good fluid handling performance, especially by using materials having particularly suitable fluid distribution properties when dry and during progressive saturation with liquids.
It is a further object of the invention to achieve this by using Super absorbent polymers.